Reducing the amount of iron ore used in the steel industry and increasing the proportion of scrap steel are key ways to achieve green and low-carbon development of the steel industry.
The “scrap steel” + “arc furnace” short process has the advantages of efficient production and green energy saving, which is of great significance to the sustainable development of the steel industry and is one of the key development directions in the future.
In the electric arc furnace steelmaking process, scrap steel is the main raw material, and its melting time accounts for more than 50% of the total time of electric arc furnace steelmaking, while the energy consumption required for its melting process accounts for more than 60% of the total energy consumption of electric arc furnace steelmaking.
Therefore, the quality of scrap steel melting characteristics has a significant impact on the smelting efficiency and smelting energy consumption of electric arc furnace.
In order to improve the efficiency of electric arc furnace steelmaking and reduce its energy consumption, metallurgical scholars at home and abroad have carried out a lot of research on the melting characteristics of scrap steel and its influencing factors during the electric arc furnace smelting stage with the help of experimental analysis technology and numerical simulation methods.
This work systematically reviews the melting characteristics of scrap steel under electric arc furnace steelmaking conditions from three aspects: hot simulation test, cold simulation test and numerical simulation. This can provide a certain theoretical reference for the subsequent study of the melting characteristics of scrap steel in the electric arc furnace steelmaking process.
Hot simulation test
Melt pool stirring properties
In order to clarify the effect of the molten pool stirring intensity on the melting behavior of scrap steel, the melting rates of pure iron and iron-carbon alloy in the iron-carbon alloy melt were studied through thermal simulation tests. The results show that the melting rate of the samples is controlled by the mass transfer in the liquid phase.
When the sample is stationary, the mass transfer process is mainly controlled by natural convection in the molten pool.
When the sample rotates, forced convection is the key factor affecting mass transfer. Afterwards, the melting behavior of scrap steel at different steel rod rotation speeds was studied. The study found that increasing the rotation speed can effectively increase the melting rate of scrap steel.
Researchers believe that efficient mass transfer and heat transfer processes are the essential reasons for the increase in scrap steel melting rate.
Figure 1 shows the relationship between steel bar rotation speed and melting rate at different molten pool temperatures.
In addition, the study found that increasing the bottom blowing gas flow rate can effectively improve the heat transfer effect between scrap steel and the molten pool.
The study examined the influence of different bottom blowing gas flow rates on the melting efficiency of steel bars. The results show that strong molten pool stirring helps to improve heat exchange efficiency and accelerate the melting of steel bars.
Scrap geometry
In terms of scrap steel size and shape, the heat transfer properties of steel bars with diameters of 30, 40 and 50 mm during the melting stage were investigated.
The results show that the increase in the diameter of the steel bar will lead to a decrease in the heat transfer coefficient between the steel bar and the molten pool.
Taking Q235 round steel as the object, the influence of steel bar diameter (20, 30, 40 and 50 mm) on the melting behavior of scrap steel was investigated.
At the same molten pool temperature, increasing the diameter of the steel bar will lead to a decrease in the convective mass transfer coefficient of carbon, thereby slowing down the melting process of the scrap steel.
The melting behavior of scrap steel when the diameter of the steel bar is 10, 20 and 30 mm was analyzed.
The reduction in the diameter of the steel bar helps to reduce the thickness of the solidified layer and shorten the melting time of the scrap steel.
When the diameter of the steel bar is 10 mm, the temperature gradient of the steel bar is small, the required heat is small, and there is no solidified layer formation process.
In addition, the melting rate of steel bars of different sizes and shapes in high-temperature molten steel is analyzed.
The study found that the smaller the sample size, the faster the melting rate of the steel bar. The difference in shape between round and square samples has little effect on the melting rate.
The effects of steel bar diameter and scrap steel specific surface area on the melting rate were studied. Reducing the diameter of the steel bar and increasing the specific surface area of the scrap steel can increase the melting rate of the scrap steel.
The effects of scrap steel size and shape (round, square and plate) on the melting behavior were also analyzed. Figures 2 and 3 show the influence of steel bar size and shape on the melting time.
In terms of the amount and type of scrap steel, the melting process of multi-level steel bars was studied, and the influence of the spacing between steel bars on the melting behavior was analyzed.
The study shows that the spacing between steel bars has a significant effect on the formation of the “steel iceberg”, which in turn controls the entire melting process. Scientific and reasonable spacing between steel bars helps to accelerate the melting process of scrap steel.
The melting process of multi-level steel bars was also studied. The study believes that the melting time of scrap steel is mainly determined by the degree of condensation between steel bars. When the spacing between steel bars increases to more than 6mm, the condensation between steel bars can be effectively inhibited, thereby improving the melting efficiency of scrap steel.
In order to explore the influence of scrap steel type on melting behavior and metal recovery, relevant work was carried out. The results show that briquetted scrap steel/slag steel has a faster melting rate than self-produced scrap steel/heavy scrap steel. At the same time, the recovery rates of heavy scrap steel, small scrap steel and briquettes are relatively high, all greater than 94%, while the recovery rates of slag steel and powder are relatively low, at 46% and 61.35% respectively.
Elemental properties of molten pool and scrap steel
In order to reveal the relationship between the carbon content of the molten pool and the melting rate, the influence of the carbon content of the molten pool on the melting process of the steel bar was analyzed with 45 steel as the object.
The study shows that a higher carbon concentration in the molten pool will increase the carbon concentration gradient between the molten pool and the steel bar, thereby accelerating the melting process of the steel bar.
After that, the influence of different molten pool carbon contents on the melting rate of the steel bar was studied. The conclusions obtained were similar.
In addition, the relationship between the carbon content in the molten pool and the mass transfer and heat transfer coefficients was studied. The study found that the mass transfer and heat transfer coefficients were not related to the carbon content in the molten pool.
In terms of the carbon content of scrap steel, the relationship between the carbon content in the steel bar and the melting behavior of scrap steel was investigated. The results show that increasing the carbon content in scrap steel will lead to a decrease in its melting point, thereby shortening the melting time of scrap steel. The effect of the carbon content in the sample on the melting rate is shown in Figure 4.
In addition, the relationship between the silicon content in steel and the melting rate was explored. The study found that an increase in silicon content will lead to a decrease in the carbon mass transfer coefficient, thereby increasing the melting time of scrap steel.
Scrap and melt pool temperature properties
Scrap preheating and melt pool temperature also have a significant effect on the melting behavior of scrap steel.
The study analyzed the melting characteristics of scrap steel at three preheating temperatures of 250, 400 and 800℃. The study found that preheating scrap steel can effectively increase its melting rate.
The relationship between scrap preheating and melting behavior was also studied. The conclusions obtained are generally consistent.
In terms of melt pool temperature, the effect of melt pool temperature on the melting rate under a series of steel rod rotation speeds was investigated. The results show that increasing the melt pool temperature can effectively increase the melting rate.
In addition, the study found that increasing the melt pool temperature has a positive driving effect on the mass transfer and heat transfer process. Afterwards, the effect of melt pool temperature changes on the melting process was studied, and similar conclusions were obtained.
Cold simulation test
At present, metallurgical workers have achieved intuitive “characterization” of scrap steel movement and melting process through cold simulation tests, and obtained quantitative or qualitative relationships between parameters such as injection flow, oxygen lance position, scrap steel size, scrap steel shape and scrap steel density and the melting properties of scrap steel, which helps to reveal the intrinsic characteristics of scrap steel melting behavior.
In order to explore the relationship between scrap steel melting and injection process, ice cubes containing potassium chloride were used to simulate scrap steel, and the effect of bottom blowing gas flow rate on scrap steel melting time was analyzed. The study found that the stirring power was positively correlated with the scrap steel melting time.
To further study the scrap steel melting behavior in the electric arc furnace steelmaking process, ice cubes were used instead of scrap steel, and hot gas was used to simulate the chemical reaction heat in the steelmaking process based on the similarity criterion, and the effects of side blowing gas flow rate and spray gun angle on ice melting time were studied.
Figure 5 shows the effect of side blowing flow rate on molten pool mixing time at different side blowing angles. It can be found that at the same injection angle, the mixing time gradually decreases with the increase of injection gas flow rate. At the same injection flow rate, reducing the injection angle is also conducive to shortening the mixing time.
In terms of scrap steel size and shape, based on the similarity criterion, the melting behavior of ice cubes of different geometric shapes and sizes under argon blowing conditions was studied, and the functional relationship between the heat transfer coefficient and the stirring work was established.
The results show that the obtained heat transfer coefficient is highly consistent with the results reported in the literature.
In order to explore the influence of various processes on the melting behavior of scrap steel, a 65t eccentric bottom arc furnace was used as a prototype to study the melting behavior of ice cubes and the mixing time of the molten pool from multiple aspects such as bottom blowing gas flow, side blowing gas flow, top gun position, ice cube size and ice cube shape.
The results show that reducing the size of ice cubes, increasing the bottom blowing and side blowing gas flow, and lowering the top gun position can all help accelerate the melting process of scrap steel and shorten the mixing time of the molten pool.
In addition, for the shape characteristics of ice cubes, spherical ice cubes are the easiest to melt, followed by cylindrical ice cubes, and square ice cubes are the most difficult, as shown in Figure 6.
Different from the method of simulating scrap steel with ice cubes, polyurethane materials are used to simulate scrap steel.
The study believes that in actual production, most scrap steel needs to be heated for a period of time before it gradually melts. However, ice cubes tend to melt quickly in the molten pool, and the traditional method of simulating scrap steel with ice cubes is difficult to accurately predict the movement behavior of scrap steel in the molten pool.
Therefore, polyurethane materials with a density close to that of water are selected to simulate scrap steel.
On this basis, the influence of factors such as the addition position, shape and density of polyurethane, the type of injection process and the injection gas flow rate on the movement behavior of polyurethane and the mixing time of the molten pool are analyzed.
In addition to electric arc furnace steelmaking, scrap steel is also used as raw material in the converter steelmaking process.
Ice blocks of different shapes and sizes are used to simulate the melting process of scrap steel in the converter steelmaking process. Through a transparent converter geometry model, the melting and movement of ice blocks can be directly observed.
Studies have shown that the melting behavior of scrap steel is closely related to the fluid flow characteristics in the converter. Strengthening the molten pool stirring is of great significance for the melting of waste materials.
After that, taking the 80t converter as the prototype, the effects of factors such as ice block shape, ice block density, ice block size, top gun position and injection gas flow rate on ice block melting behavior and molten pool mixing time were analyzed.
The results show that reducing the oxygen gun position, reducing the ice block size, reducing the ice block density, and increasing the injection gas flow rate are all conducive to promoting the ice block melting process and shortening the molten pool mixing time.
